It is not true that slippery slopes are always and necessarily fallacies.
Sometimes the phrase “slippery slope” is just a manipulative term to describe what might otherwise be called a precedent.
Or it could be a deliberately constructed slippery slope – also known as a wedge strategy.
The fallacy in a slippery slope argument occurs when and only when there is no link between the initiating event and the catastrophic outcome(s) – when it is or ought to be self-evidently absurd that a butterfly in Tokyo cannot possibly cause a hurricane in New York.
TWO roads diverged in a yellow wood,
And sorry I could not travel both
And be one traveler, long I stood
And looked down one as far as I could….
From The Road Not Taken by Robert Frost
The cry of “slippery slope fallacy!” tends to be used by lazy thinkers who can’t or won’t take the time and trouble to evaluate the argument to the point where they can actually pinpoint the error. They think it is enough to say “there’s no reason to suppose X” (where X is the relationship linking initiating event A to outcome B), as if that proved the issue. That is itself a fallacy, because what is, in effect, happening, is that someone is trying to use the claim that X is not true to prove the claim that X is not true – in other words, begging the question.
The correct response to a slippery slope argument is to identify the actual flaw in the reasoning – to identify and define X (the point where the argument suggests A will lead to B), and to prove X contains (“is hiding”) a logical flaw. You have to actually prove that the actual implication is not supported.
If the actual implication is supported, then it’s not a fallacy.
What’s ridiculous is not the idea that sometimes certain rules or decisions can and do lead to avalanche effects. What’s ridiculous is that for over a hundred years we’ve been dismissing such arguments. The issuance of social security cards did in fact lead directly to every citizen being identified by number. And while one could easily say those fussy old ladies in 1915 were being unpleasantly prudish when they complained about Charlie Chaplin’s misbehaving walking stick (and his tendency to look up skirts), can anyone really say they were wrong when they claimed that allowing film artists a license to trespass the boundaries of the socially acceptable would lead straight to a future where sex, crime, and violence in movies become normal and socially acceptable?
What actually happens here is that we know the ladies have a point, but we pretend they were being ridiculous because it’s just easier than actually articulating what we really believe. What we really believe is that there is something beneficial about allowing artists to trespass social boundaries. We believe it protects us from stagnation or cultural repression. The problem with skipping the part where we defend what we do believe, though, is that we don’t set the rule as it should be, based on our goals and our expectations (and thus we can’t evaluate the outcomes later). Instead we simply neglect to set any rules at all – kicking the question down the road for future generations to worry about. This procrastination has a price, in the form of unintended consequences.
The history of the 20th century is full of “slippery slopes” that came true after those who predicted them were ridiculed. The reason is because our culture is a complex system: its variables are interconnected, and changing one variable tends to change every other variable in some way.
But of course it’s much easier to simply say, “Aha! Your argument suggests A will lead to B! That construction couldn’t possibly be valid, therefore I win!” The problem with that is that it’s not true: sometimes A really does lead to B.
Those who wish to employ slippery slope constructions, however, are advised to identify and be prepared to defend the relationship X that combines with A to cause B.